1. Field of the Invention
The invention relates to well testing in general and in particular to a method for downhole measurements and recording of data from a multiple layered formation of an oil and gas well and for estimating individual permeabilities and skin factors of the layers using the recorded data.
2. Description of the Prior Art
The estimation of parameters of stratified layers without crossflow of an oil and gas well is not a new problem. Over the years, many authors have investigated the behavior of layered reservoirs without cross flow. Much work has been done on estimating parameters of layered reservoirs because they naturally result during the process of sedimentation. Layered reservoirs are composed of two or more layers with different formation and fluid characteristics.
One of the major problems for layered reservoirs is the definition of the layers. It has been found that it is essential to integrate all logs and pressure transient and flowmeter data in order to determine flow capacities, skin factors, and the pressure of individual layers. This invention relates primarily to two-layer reservoirs with a flow barrier between the layers (without crossflow). The production is commingled at the wellbore only.
During a buildup test, fluid may flow from the high pressure zone to the low pressure zone through the wellbore as a result of a differential depletion. The crossflow problem becomes more severe if the drainage radius of each zone is different. Wellbore crossflow could occur while the pressure is building up. A straight line may be observed on the Horner plot. This behavior has been observed many times in North Sea reservoirs.
The crossflow problem has been overlooked by the prior art because in many instances the pressure data itself does not reveal any information about the wellbore crossflow. Furthermore, the end of the wellbore crossflow between the layers cannot be determined either quantitatively or qualitatively. If fluid segregation in the tubing and wellbore geometry is added to the complication mentioned above, the buildup tests from even two-layer reservoirs without crossflow cannot easily be nalyzed.
This invention relates to the behavior of a well in an infinite two-layered reservoir. If the well has a well-defined drainage boundary (symmetric about the well axis for both layers), and if a well test is run long enough, the prior art has shown that it is possible to estimate the individual layer permeabilities and an average skin. However, cost or operational restrictions can make it impractical to carry out a test of sufficient duration to attain a pseudo steady-state period. Moreover, even if the test is run long enough, an analyzable pseudo steady-state period may not result because of non-symmetric or irregular drainage boundaries for each layer. It is also difficult to maintain a constant production rate long enough to reach a pseudo steady-state period.
A major problem for layered systems not addressed by the prior art is how to estimate layer permeabilities, skins, and pressures from conventional well testing. In practice, the conventional tests (drawdown and/or buildup) only reveal the behavior of a two-layer formation which cannot be distinguished from the behavior of a single-layer formation even though a two-layer reservoir has a distinct behavior without wellbore storage effect. There are, of course, a few special cases for which the conventional tests will work.
The effect of wellbore storage on the behavior of the layered reservoirs is more complex than that of single-layer reservoirs. First, the wellbore storage may vary according to the differences in flow contribution of each layer. Second, it has been observed that it takes longer to reach the semilog straight line than that of the equivalent single-layer systems.
It is important for the operator of an oil and gas well having a multiple layer reservoir to be able to determine the skin factor, s, and the permeability, k, of each layer of the formation. Such information aids the operator in his determination of which zone may need reperforation or acidizing. Such information may also aid the operator to determine whether loss of well production is caused by damage to one layer or more layers (high skin factor) as distinguished from other reasons such as gas saturation buildup. Reperforation or acidizing may cure damage to the well while it will be useless for a gas saturation buildup problem.